The invention relates to a power-split gear, consisting of a hydrostatic unit and of a mechanical gear unit containing a summing gear and range gear, said hydrostatic unit and gear unit being accommodated in a common case with a cover, the mechanical gear unit being arranged on the case cover, and being capable of being inserted as a closed subassembly into the case. The mechanical gear unit constitutes the mechanical branch, the hydrostatic unit the hydrostatic branch.
Power-split gears having the components mentioned are high-efficiency continuously variable gears which, above all in tractors, are intended to replace the conventional multistep gears having 30 shift steps or more. They differ fundamentally from the latter; the summing gear is a complex planetary gear and the range gears are shifted by means of hydraulically actuated clutches or brakes. They are therefore highly complicated both mechanically and in terms of their control. Gears of this type are described, for example, in DE-A1 40 27 724, 44 01 509 and 195 25 823, as well as in U.S. Pat. No. 4,706,806.
Installing their components in the case or exchanging them and conducting an operating test on them is therefore difficult and labor-intensive, above all as regards a tractor gear, the case of which is screwed to the engine block and rear axle forming the basic structure of a tractor. In addition, numerous fluid connections, which are usually made by means of hoses and pipes, are required for lubrication and control and for the hydrostatic unit, and the latter, because it generates noise, needs a suspension which decouples structure-borne sound. All this makes assembly even more difficult and the connecting parts make the gear susceptible to faults.
Admittedly, it is known from EP 115 794 A1, in power-shiftable toothed change gears of conventional type, to insert a layshaft, together with its couplings, as a closed subassembly into the case from outside through an orifice closable by means of a cover, for which purpose the cover may be provided with carrying arms. However, the layshaft is only an individual shaft which can easily be mounted at its two ends and otherwise cooperates only with the mutually coaxial main gear shafts, namely the input and the output shaft.
Furthermore, it is known from U.S. Pat. No. 5,345,839 to suspend a hydrostatic (and therefore also fundamentally different) gear in a case by means of transverse rods so as to be decoupled from structure-borne sound. The gear can consequently be inserted as a subassembly into the case from outside, but it is difficult to make the connections necessary for control and the transverse rods make it virtually impossible to accommodate further gear elements in the case.
The object of the invention, therefore, is to design the case of a power-split gear in such a way that the components of the gear can be mounted and demounted easily at the least possible outlay in terms of construction and its components are simple to exchange, and in such a way that there are as few parts susceptible to faults as possible and test runs can be carried out prior to installation.